Apparatus employing negative resistance device for operating electric discharge lamps



3,165,668 Patented Jan. 12, 1965 3,165,665 AP?ARATUS EMPLGYHNG NEGATHVEBR55181 ANCE DEVTQE GPERATKNG ELEQTRTQ DISCHARGE LAP/E 1 5 Theodore it."piey, fianviiie, 113., assignor to General Electric 'Company, acorporation oi New York Filed Dec. 17, 1962, Ser. No. 245,122

This invention relates to apparatus for operating electric dischargelamps and more particularly to such an improved apparatus employingsolid state devices.

In conventional ballasts an impedance element is usually provided inseries with the electric discharge lamp in order to limit the lampcurrent to a proper value after the lamp is started. Where the electricdischarge lamp is operated from an alternating power source, theimpedance element may introduce into the lamp circuit an inductivereactance or the combination of inductive and capacitive reactance toprovide the ballasting action. In applications where an electricdischarge lamp is operated with direct current, the current supplied tooperate the lamp is generally limited by placing a resistor in seriescircuit with the electric discharge lamp.

In an application Serial Number 192,231, filed on May 31, 1962, in thename of Walter F. Powell, Jr., and assigned to the same assignee as thepresent invention, there is described and claimed an arrangement foroperating electric discharge lamps wherein the electric discharge lampsare ballasted by repetitively switching the current supplied to thelamps between controllable levels. The present invention relates to animproved arrangement employing one transistor and tunnel diode or theequivalents thereof in the lamp operating circuit. The ballasting of thelamp is accomplished by repetitive switching action wherein the energydelivered to the lamp and stored in the magnetic field of an inductor iscontrolled.

Accordingly, a general object of the present invention is to provide animproved ballast apparatus for operating electric discharge lampswherein the ballasting action is achieved by a repetitively switchingaction to control the energy delivered to the lamp and stored in aninductor.

It is another object of the invention to provide an improved apparatusfor operating electric discharge lamps employing relatively few solidstate devices wherein current control is achieved by the fast switchingaction of the solid state devices.

In accordance with one form of my invention I have provided an improvedapparatus for supplying energy from a source of potential to operate oneor more electric discharge lamps. The apparatus includes a firstsemiconductor switching means such as 'atransistor, a secondsemiconductor means characterized by a negative conductance, forexample, a tunnel diode, and an energy storage element such as aninductor, all of which are connected in series circuit relation with thelamp.

The lamp is ballasted by the repetitive switching action of the firstsemiconductor means, which causes the lamp current to be maintainedwithin predetermined levels.

The switching action is controlled by the second semiconductor meanswhich causes the first semiconductor means to be activated from aconductive to a nonconductive state as the second semiconductor meansshifts from a high voltage to a low voltage state. To insure that energyis supplied to the lamp during the nonconductive interval of the firstsemiconductor means, energy stored in the energy storage element isdelivered to the lamp during this interval. In a prefered embodiment ofthe invention a diode is connected in parallel circuit relation with thelamp and in circuit with the inductive energy storage element, toprovide a path for the energy stored in the magnetic field of theelement to be delivered to the lamp when the first semiconductor meansis nonconductive.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. My invention, however, as to organization and method ofoperation together with further objects and advantages thereof may bestbe understood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIGURE 1 is a schematic circuit diagram of a ballast apparatus embodyingmy invention; and

FIGURE 2 is a schematic diagram of an apparatus embodying the improvedarrangement of my invention wherein a starting circuit and filamenttransformer are provided.

Referring more particularly to the schematic circuit diagram shown inFIGURE 1, the improved apparatus 16 for supplying an operating potentialto an electric discharge lamp 11, is shown enclosed in a dashedrectangle 12 which schematically represents the housing means for thecomponents of the apparatus 10. is adapted for operation from a DC.source, input lead 13 being adapted for connection to the positive sideof the source and input lead 14 being adapted for connection to thenegative side. I

To provide the fast switching action required for controlling theoperation of the lamp 111, a transistor Q is provided in a circuit withlead 14, a tunnel diode TD and inductor L and an output lead 15connected in circuit with one end of the lamp 11. It will be seen thatthe other end of lamp 11 is connected in circuit with the positive sideof the power source by means of lead 13. The transistor Q, the tunneldiode TD and the inductor L which serves as an energy storage element,are con nected in series circuit relation with lamp 11. i

A path for the inductive decay current from inductor L when thetransistor Q; is switched to a nonconductive state, is provided by aunidirectional conducting device, such as a diode D which is connectedto the lead joining the emitter of transistor Q with the tunnel diode TDand to input lead 13; The diode D is poled so that it does not conductwhen'power is being supplied from the power supply to lamp 11. Dependingupon the state of the tunnel diode TD the transistor Q is renderedconductive by a bias signal from potential source 16, which is coupledwith the transistor Q and the tunnel diOde TD1.

The transistor Q used in the exemplification of the invention was apower PNP transistor connected in. a common emitter circuitconfiguration. The PNP transistor is comprised of three zones ofsemiconductive material forming two PN junctions, the base being joinedto the N-zone, and the emitter and collector being joined to the outerP-zones. The transistor Q is switched from a conductive to anessentially nonconductive state by cancelling the bias signal whichprovides the base drive for transistor Q The tunnel diode ,TD used inthe exemplification of the invention Was a two terminal semiconductordevice employing a single PN junction. The electrode joined to theN-layer is referred to herein as the cathode and is representedschematically in the drawing by the channel shaped symbol. The electrodejoined to the P-layer of the tunnel diode TD is referred to herein asthe anode and is represented schematically in the drawing by the arrowsymbol.

When the current level through the tunnel diode TD, is less than thepredetermined peak point value, the tunnel diode TD, exhibits a lowresistance and may be considered to be in a low impedance or low voltagestate. insofar as the tunnel diode TD is concerned, the circuit Theapparatus 10 has constant current properties; that is, the impedance ofthe tunnel diode TD does not at any time influence the lamp currentexcept that the voltage drop across it is used as an indication of thelamp current. This voltage drop is used to produce. the switching actionin the transistor Q by a process of non-linear current feedback or byemitter degeneration of the transistor Q Thus, each time the tunneldiode TD switches, it does so under conditions of constant current.

It will be appreciated that the apparatus of the present invention isintended to operate the electric discharge lamp 11. If the transistor Qhad a higher voltage rating, a D0. supply of 200 volts could have beenused to start and operate the fluorescent lamp 11 of the illustratedembodiment of the invention. Since the apparatus 1-0 was operated from a60 volt D.C. supply, a starting circuit was used to initially fire thelamp. In FIGURE 2 I have shown an arrangement for starting an electricdischarge lamp 11 by placing it initially across a source of alterhatingcurrent. A resistor R was provided to limit the current during thestarting condition. Since lamp 29 was a hot cathode type of fluorescentlamp, a filament transformer T having a primary P and a pair of heatingwindings H and H was employed to supply a heating current to the lampfilaments.

It will be seen that the lamp operating circuit shown in 'FIGURE 2 isessentially the same as the operating circuit of FIGURE 1. Accordingly,I have used the same reference symbols to identify the correspondingparts of the operating circuit. The input leads 13, 14 in the schematiccircuit diagram of FIGURE 1 are shown connected in circuit with a sourceof direct current potential, which is schematically shown as a battery17. In order to switch lamp 20 overto the starting circuit, a doublepole', double throw switch 18 was employed.

When the switch 18 engages the contacts 21, 22, the lamp 20 is placed incircuit across an alternating current source, such as a 60 cycle, 120volt supply, to which leads 25, 26 are normally connected. As willhereinafter he more fully described the 24 inch T-12 High Outputfluorescent lamp 20 used in the illustrated embodiment of the inventioncould be started directly from the 120 volt supply. It will beunderstood, of course, that longer lamps will usually require a highersource of starting potential.

When switch 18 is placed, in position so that the contacts 23 and 24 areengaged, the lamp 20 is connected in circuit with the operating circuitand is disconnected from the alternating current source except for thecathode heating current supplied by transformer T It will be seen thatin either position of the switch 18, the filaments of fluorescent lamp20 are supplied with heating current, andthe filaments of lamp 20 areconnected by leads 27, 28, 29 and 30 with the filament heating windingsH and H By way of illustration the apparatus shown in FIG- URE l wasreduced to practice.v The following circuit components were used tooperate a 24 inch T-12 High Output fluorescent lamp:

Transistor Q 2N1906 PNP power transistor.

Tunnel diode TD; STD-612 tunnel diode (General Electric).

Diode D 4JA10D diode (General Electric).

Inductor L Ferrite core inductor, 45

millihenries.

Supply potential source..- 60 volts D.C.

Bias voltage source 0.5 volt D.C.

The lamp 11 was started initally by connecting the lamp directly acrossa 120 volt, 60 cycle power source as shown in FIGURE 2. The smallfilament transformer T having its primary P connected to the powersource, was used to supply cathode heating current. After the lamp 11was ignited, the lamp 11 was operated from the 60 volt D.C. supply at afrequency of 1000 cycles per second.

Referring now to FIGURES 1 and 2, the operation of the operating circuitwill be more fully described. Referring more specifically now to FIGURE2, the lamp 20 was ignited by switching it across the leads 25, 26 whichwere connected to an alternating current source. After the lamp wasignited, it was switched to the DC. source 17. Until the current throughthe lamp 20 reaches the peak point value of the tunnel diode TD it willbe understood that the voltage drop across the tunnel diode isnegligibly small. For the tunnel diode used in the exemplification ofthe invention, the voltage drop was approximately 60 millivolts.However, when the lamp current reaches the peak current value of thetunnel diode TD it switches to the high voltage state. In the highvoltage state the voltage drop across the tunnel diode TD, increases inmagnitude. For the tunnel diode TD, used in the exemplification of theinvention, this voltage drop was approximately 0.5 volt. Since thepolarity of the voltage drop across the tunnel diode TD is opposite tothe bias potential, the effect of the 0.5 volt bias potential is therebyeffectively cancelled out. Consequently, the transistor Q is renderednonconductive.

For this condition of the circuit, the polarity of the voltage acrossthe inductor L reverses, and the potential at point A shown in FIGURE 1is now more positive than the potential at input lead 13. Diode D isforward biased and provides a path for the inductive decay current fromthe inductor 1. When the lamp current reaches the valley current valueof the tunnel diode TD tunnel diode TD switches to the low voltagestate. The voltage drop across the tunnel-diode TD falls off to anegligibly small value, and the bias potential again becomes effectiveto drive the transistor Q Thus, transistor Q is rendered conductive, anddiode D reverts to a blocking state. Cur rent is now supplied to thelamp 20 from the DC. power source 17, and another cycle of operationbegins.

The current supplied to the lamp from the power source is effectivelylimited since the transistor Q is switched off to block any excursion ofcurrent from the power source when the lamp current reaches the peakpoint value of the tunnel diode TD During the interval in which thetransistor Q is switched oil, the lamp 11 is supplied with a decayingcurrent from the inductor L connected in series with the lamp. The lampcurrent is not allowed to drop below the valley point value of thetunnel diode TD since the transistor Q is again switched on whenelectric discharge lamp 1]. is effectively operated by a chopped currentthat oscillates between the peak point and valley point values of thetunnel diode TD Referring to the circuit diagram of FIGURE 1, it will beseen that during the conductive state of transistor Q1, the path ofcurrent flow is essentially from the positive input 'lead 13, to lamp11, the inductor L the tunnel diode TD the transistor Q and to thenegative input terminal lead 14. When the lamp current reaches the peakpoint value of the tunnel diode TD the tunnel diode TD switches to itshigh voltage state, and the voltage drop across the tunnel diode TDcancels the potential across the source 16 to render transistor Qnonconductive. During the high voltage state of the tunnel diode TD thecontinuity of the supply of energy to the lamp 11 is maintained by thedelivery of the energy stored in the magnetic field of the inductor LWhen the lamp current decays to the valley point value of the tunneldiode TD the tunnel diode TD switches to its low voltage state tothereby cause transistor switch Q to be again rendered conductive. Thisswitching cycle is repeated to provide alternate excursions of energyfrom the power source and the inductor L the current level switchingeffectively limiting the lamp current.

From the foregoing description of the exemplification of my invention,it will be apparent that a solid state apparatus utilizing relativelyfew components such as a tunnel diode and power transistor can beemployed to operate electric discharge lamps. An important advantage ofthe ballast apparatus of the invention is that the fluorescent lamp 11can be operated with a relatively high efficiency. The only losses inthe apparatus are the small losses in the semiconductor devices D Q andTD and the copper and radiation losses in the inductor L It will beappreciated that where the lamp 11 is operated at a frequency above 100kilocycles, the self-inductance of the arc in the fluorescent lamp 11would serve as a suflicient energy storage means, and the externalinductor L would not be required.

It will be apparent to those skilled in the art that semiconductordevices other than those described in the exemplification of theinvention can be adapted to perform the functions in accordance with thepresent invention and that many other modifications of the invention maybe made. Therefore, I intend by the appended claims to cover all suchmodifications that fall within the true spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An apparatus for supplying energy from a source of potential to anelectric discharge lamp, said apparatus comprising a first semiconductormeans having a number of zones of semiconductive material forming atleast a pair of PN junctions, said means being rendered conductive by abias signal, a second semiconductor means characterized by sharpimpedance shift when operated under predetermined current conditions,said second means switching to a high voltage state in response to afirst predetermined value of current passing through said second meansand switching to a low voltage state when the current through saidsecond means decreases to a second predetermined value, bias meanscoupled with said first and second means for applying a bias signal torendersaid first means conductive, an inductive energy storage means,and circuit means for connecting said first and second semiconductormeans and said storage means in circuit with the lamp and the potentialsource, said circuit means including a unidirectional conducting deviceto provide a path for the delivery of energy stored in the inductiveenergy storage means to the lamp when said first means is renderednonconductive, said first semiconductor means being rendered conductiveby said bias means until the current through said second semiconductormeans reaches said first predetermined value whereby said second meansis switched to a high voltage state and cancels the bias signal torender said first means nonconductive,

2. An apparatus for operating an electric discharge lamp from a powersource, said apparatus comprising a semiconductor means, a tunnel diodeconnected in series circuit relation with said semiconductor means, apotential source coupled with said semiconductor means for renderingsaid semiconductor means conductive, said potential source beingconnected in circuit with said tunnel diode, said potential of saidsource being cancelled when said tunnel diode is switched to a highvoltage state and being applied at said semiconductor means when saidtunnel diode reverts to a low voltage state, said supply of energy fromthe power source to the lamp being interrupted when said semiconductormeans is rendered nonconductive, an energy storage element, and circuitmeans for connecting said semiconductor means, said tunnel diode andsaid energy storage element in circuit with said lamp, said circuitmeans providing a path for the storage of energy in said element duringthe supply of operating potential from the source to the lamp and fordelivery of said energy stored in said element to the lamp when saidsupply of operating potential from the source to the lamp is interruptedby said semiconductor means.

3. An apparatus for operating a fluorescent lamp from a source ofpotential, said apparatus comprising a transistor, a tunnel diode, aninductor, circuit means for connecting said transistor, said tunneldiode and inductor with the source of potential in circuit for operatingthe lam said means including a unidirectional conducting deviceconnected in circuit with said inductor and said tunnel diode to providea path for energy released from said inductor when the transistor isrendered nonconductive, means coupled with said transistor to apply abias potential thereto, said tunnel diode being switched to a highvoltage state when the current therethrough reaches a firstpredetermined level thereby cancelling said bias potential and renderingsaid transistor nonconductive, said tunnel diode switching to a lowvoltage state when the current therethrough decreases to the secondpredetermined level, whereby said apparatus operates said lamp byrepetitively causing the current supplied to the lamp to vary betweensaid first and second predetermined levels.

4. An apparatus for operating an electric discharge lamp from a sourceof direct current potential, said apparatus comprising a transistor, abias potential source coupled with the transistor for rendering saidtransistor conductive, a tunnel diode, an inductor and circuit meansincluding an input for connection with the source of potential andconnections for placing said transistor, said tunnel diode and inductorin series circuit relation with the lamp, said circuit means includingmeans to provide a path for the supply of energy from the inductor tothe lamp when said transistor is rendered nonconductive, said tunneldiode causing the bias potential to be cancelled when the currenttherethrough reaches a first predetermined level thereby rendering thetransistor nonconducting and said tunnel diode being switched to a lowvoltage state when the current therethrough decreases to a secondpredetermined level thereby rendering said transistor conductive.

References Cited in the file of this patent UNITED STATES PATENTS3,060,348 Todd Oct. 23, 1962

1. AN APPARATUS FOR SUPPLYING ENERGY FROM A SOURCE OF POTENTIAL TO ANELECTRIC DISCHARGE LAMP, SAID APPARATUS COMPRISING A FIRST SEMICONDUCTORMEANS HAVING A NUMBER OF ZONES OF SEMICONDUCTIVE MATERIAL FORMING ATLEAST A PAIR OF PN JUNCTIONS, SAID MEANS BEING RENDERED CONDUCTIVE BY ABIAS SIGNAL, A SECOND SEMICONDUCTOR MEANS CHARACTERIZED BY SHARPIMPEDANCE SHIFT WHEN OPERATED UNDER PREDETERMINED CURRENT CONDITIONS,SAID SECOND MEANS SWITCHING TO A HIGH VOLTAGE STATE IN RESPONSE TO AFIRST PREDETERMINED VALUE OF CURRENT PASSING THROUGH SAID SECOND MEANSAND SWITCHING TO A LOW VOLTAGE STAGE WHEN THE CURRENT THROUGH SAIDSECOND MEANS DECREASES TO A SECOND PREDETERMINED VALUE, BIAS MEANSCOUPLED WITH SAID FIRST AND SECOND MEANS FOR APPLYING A BIAS SIGNAL TORENDER SAID FIRST MEANS CONDUCTIVE, AN INDUCTIVE ENERGY STORAGE MEANS,AND CIRCUIT MEANS FOR CONNECTING SAID FIRST AND SECOND SEMICONDUCTORMEANS AND SAID STORAGE MEANS IN CIRCUIT WITH THE LAMP AND THE POTENTIALSOURCE, SAID CIRCUIT MEANS INCLUDING A UNIDIRECTIONAL CONDUCTING DEVICETO PROVIDE A PATH FOR THE DELIVERY OF ENERGY STORED IN THE INDUCTIVEENERGY STORAGE MEANS TO THE LAMP WHEN SAID FIRST MEANS IS RENDEREDNONCONDUCTIVE, SAID FIRST SEMICONDUCTOR MEANS BEING RENDERED CONDUCTIVEBY SAID BIAS MEANS UNTIL THE CURRENT THROUGH SAID SECOND SEMICONDUCTORMEANS REACHES SAID FIRST PREDETERMINED VALUE WHEREBY SAID SECOND MEANSIS SWITCHED TO A HIGH VOLTAGE STATE AND CANCELS THE BIAS SIGNAL TORENDER SAID FIRST MEANS NONCONDUCTIVE.